Organic light emitting diodes (OLEDs) are becoming more widely used in displays and other optoelectronic applications. Organic electronic displays typically consist of a matrix of OLEDs, each of which comprises thin films of organic materials that emit light when excited by an electric current. The organic thin films are typically sandwiched between an anode and a cathode, which provide an electric current to the organic thin film to enable the film to emit light. In a display, the light emitted by the organic thin film must exit the thin film and penetrate through at least one of the electrodes to be visible to a user. Hence, at least one of the electrodes in the electrode pair comprises a transparent conductor such as a transparent conducting oxide (TCO).
Indium tin oxide (ITO) is the most commonly used TCO due to its transparency and its high conductivity relative to other TCOs. ITO is used in various applications requiring transparency and conductivity including liquid crystal displays, plasma displays, photovoltaics, electronic ink displays, and OLED displays. ITO is typically deposited as a thin film on a transparent substrate such as glass.
In the context of OLEDs, an ITO layer is typically formed on a transparent substrate used as the anode. Holes are injected from the anode into a hole transport layer (HTL), which carries the holes to the light emitting thin film layer. Concurrently, electrons are injected via the cathode and are transported through the electron transport layer (ETL) and recombine with the holes in the light emitting thin film layer to release a photon. The photon emitted in the thin film layer may then escape the thin film layer, pass through the HTL and exit the OLED device through the ITO layer and the transparent substrate.
The energy required to inject holes from the anode is dependent on the hole injection barrier height. The hole injection barrier height depends on the difference between the work function of the anode and the highest occupied molecular orbital (HOMO) of the adjacent organic layer. The hole injection barrier of existing OLEDs is high but this can be mitigated by providing one or more intermediate organic layers. Each organic layer has a subsequently deeper HOMO level, enabling holes to pass through a larger number of smaller injection barriers rather than a single large injection barrier. However, each additional organic layer increases the cost of the device and decreases the yield of the manufacturing process.
It is an object of the present invention to mitigate or obviate at least one of the above disadvantages.